This application is cross-referenced to application Ser. No. 09/816,981 entitled xe2x80x9cDistributed Converter And Method For Equalizing Lithium-ion Batteriesxe2x80x9d that is being filed simultaneously with the present invention.
This invention relates to a redundant AC power system and, more particularly, to such a power system useful in spacecraft applications.
The source of the power in communications satellites and many other spacecraft is solar cells. The solar cells generate DC (direct current) power that is either used directly or stored in batteries that subsequently provide DC power to the various power loads in the spacecraft. DC power systems are fully satisfactory for many applications.
However, as power requirements in such spacecraft become ever-larger, it is contemplated that multiphase AC (alternating current) power will be required to deliver the requisite power levels. The generation of AC power from DC power sources is well known, and power converters are available. The use of AC power systems on spacecraft having fundamentally DC power sources such as solar cells is quite feasible.
One of the primary requirements for spacecraft having long-duration missions, such as the 15-year or more design life of a communications satellite, is fault tolerance achieved through carefully selected redundancy. The various components of each system are designed for high reliability and often have predicted lives exceeding this design life. Nevertheless, care is taken to provide redundancy so that, if a single component of any system fails, there will not be a catastrophic failure of the spacecraft.
In the case of an AC power system, it is expected that there would be multiple AC power sources to fulfill the requirements of redundancy. These multiple sources are connected in parallel to a power bus that delivers the power the various loads. If one of the AC power sources were to fail to a shorted state, it would short the entire system so that the one failure would lead to catastrophic failure of the power system.
Provision would therefore be made to remove any electrically shorted AC power source from the system before it could lead to such a result. The most straightforward approach to removing the electrically shorted AC power source is an arrangement of relays or switches on each leg of each of the AC power sources, and sensors for detecting electrical shorting. If an electrical short is sensed in one of the AC power sources, its switches or relays would be immediately operated to remove it from the source system. The current capabilities of the power system would be reduced by the current supplied by the failed power source that is removed from the power system, but the power system would continue to operate with the reduced capability.
This approach would be operable, but switches, relays, and their associated sensors and controls are heavy and bulky. Weight and volume additions to the spacecraft carry a heavy launch penalty, utilizing launch capability that could otherwise be used for other portions of the spacecraft payload. Switches and relays also are subject to degradation over time and have limited dielectrical withstanding voltage.
There is therefore a need for an AC power system that is fully redundant, but in which the features that accomplish the redundancy are of minimal weight and size. The present invention fulfills this need, and further provides related advantages.
The present invention provides an AC power system with full redundancy. The AC power system is of particular value in spacecraft applications, but it also may be used in other AC power systems. The present technique achieves redundance in a fully passive manner. No switches, relays, sensors, controls, or other heavy components are required to achieve the redundancy.
In accordance with the invention, a redundant AC (alternating power) power system comprises a first AC power source comprising a first AC voltage supply having a pair of first-supply outputs. The first AC power source has a first-source transformer with a first-source transformer primary winding having a pair of first-source transformer primary inputs each connected to a respective one of the first-supply outputs, and a first-source transformer secondary winding having a first-source transformer first output, a first-source transformer second output, and a grounded center tap. The first AC power source also includes a first-source first-output diode having an input (anode) connected to the first-source transformer first output, and an output (cathode), and a first-source second-output diode having an input (anode) connected to the first-source transformer second output, and an output (cathode). There is a similar second AC power source comprising a second AC voltage supply having a pair of second-supply outputs. The second AC power source has a second-source transformer with a second-source transformer primary winding having a pair of second-source transformer primary inputs each connected to a respective one of the second-supply outputs, and a second-source transformer secondary winding having a second-source transformer first output, a second-source transformer second output, and a grounded center tap. The second AC power source also includes a second-source first-output diode having an input (anode) connected to the second-source transformer first output (cathode), and an output, and a second-source second-output diode having an input (anode) connected to the second-source transformer second output, and an output (cathode). A load transformer has a load transformer primary winding with a pair of load transformer primary inputs and a load transformer primary center tap connectable to ground. A first one of the load transformer primary inputs is connected to the output of the first-source first-output diode and to the output of the second-source first-output diode. A second one of the load transformer primary inputs is connected to the output of the first-source second-output diode and to the output of the second-source second-output diode. Optionally but desirably, a switch is connected between the center tap of the load transformer and ground.
The load transformer preferably has a secondary winding with a pair of load transformer outputs, and wherein the power system further includes a load having two inputs, each of the load inputs connected to a respective one of the load transformer outputs. An advantage of this approach is that the power system has no switch or relay between the first AC power source and the load, and no switch or relay between the second AC power source and the load.
In one application, the first AC power source and the second AC power source are each DC-to-AC power converters. An output frequency of such first and second AC power sources is typically, but not necessarily, from about 1 kilohertz (KHz) to about 100 KHz. The first AC power source and the second AC power source may each be DC-to-AC power converters operating from a DC source such as a solar cell or a battery. In an application of particular interest, the battery may be a lithium-ion battery source.
As long as an AC voltage source is operable, the diodes permit power to flow in the two legs. If one of the AC voltage sources fails to an open-circuit condition, power from the other, operable AC voltage source(s) cannot flow to the failed voltage source because of the diodes, which thereby provide a diode steering path. No switches or relays are required to prevent the flow of power to the failed AC voltage source.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.